RF-resonator tuning

ABSTRACT

A RF-resonator ( 1 ) comprises a resonator body ( 2 ) with at least one resonant cavity ( 3 ). A conductive sheet material forms a surface area of the resonant cavity ( 3 ), the conductive sheet material being deformable by application of a mechanical force for tuning the resonator to a target frequency. A method for tuning the RF-resonator, comprising the following steps: measuring the resonance frequency of the resonant cavity ( 3 ), and deforming the conductive sheet material by applying an increasing mechanical force to the conductive sheet material until the target frequency of the resonant cavity ( 3 ) is reached.

The invention is based on a priority application EP 05 290 294.7 whichis hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a RF-resonator comprising a resonator body withat least one resonant cavity and a method for tuning the same.

BACKGROUND OF THE INVENTION

Electronic systems which use extremely high radio frequencies (RF), suchas optical data transmission equipment or millimeter wave radio, eitherpoint-to-point or point to multipoint, as well as telematic equipmentand subscriber access systems require resonators and filters with aprecisely adjusted resonator frequency.

To assure tunability of manufactured raw filter components to a desiredexact target frequency, the state of the art makes use of specialmicrowave tuning components (e.g. metallic screws/dielectric rods).These components are commonly available on the market, but expensive.Their price is caused by gold-plating, choice of specialized materials,labor cost for mounting the screw/nut/rod subassembly as well as veryexpensive means to assure good and constant long-term (30 years) contactfrom screw to nut all around the screw's circumference, e.g. byintroduction of individual spring-loaded slits in the screw. Typicalmaximum operating frequencies specified are around 12 GHz, with someexceptions up to 18 GHz.

Going to higher and higher frequencies, standard tuning componentsintroduce more and more problems due to their increasing relative sizeto wavelength ratio. On the other hand, screw manufacturing costsincrease dramatically when trying to keep screw diameter small comparedto free-space wavelength (e.g. 3.5 mm for 85 GHz) with manufacturingtolerances scaled down as well. The state-of-the-art components becomemore and more fragile to handle as well.

OBJECT OF THE INVENTION

It is the object of the invention to provide a RF-resonator and a methodfor tuning the RF-resonator to a target frequency in a straightforward,repeatable and cost-effective way.

SHORT DESCRIPTION OF THE INVENTION

This object is achieved by a conductive sheet material forming a surfacearea of the resonant cavity, the conductive sheet material beingdeformable by application of a mechanical force for tuning the resonatorto a target frequency.

The inventive RF-resonator separates the functionalities of ‘forcedmechanical movement’ and ‘electromagnetic field manipulation’ which aretraditionally combined in one component. In such a way any, even minor,gaps in the resonator are avoided as would exist between threaded parts.By applying a mechanical force on the conductive sheet material, anelastic or inelastic deformation of the sheet material is generatedwhich changes the electromagnetic field inside the cavity in order tocompensate for manufacturing tolerances of the cavity.

In a preferred embodiment, the conductive sheet material is a metalfoil. Metal foils are inexpensive materials which can easily be deformedby applying a mechanic force.

In a preferred embodiment, a face of the resonator body is covered bythe conductive sheet material. In this way, the surface area of theresonant cavity is covered and at the same time a sufficient amount ofspace is provided for fixing the sheet material on the resonator body.

In a further preferred embodiment, the conductive sheet material isfixed on the resonator body by a cover mounted on the resonator body. Itis extremely important that even minor gaps in the resonator cavity areavoided. As the conductive sheet material forms part of the cavitysurface, it is also important to protect it from external forces leadingto unwanted deformations causing a change in the target frequency of theresonator.

In a further embodiment, threaded holes are formed in the cover and theresonator body for mounting the cover on the resonator body. The covercan be fixed to the resonator body by inserting screws in the holes,such that unwanted deformations of the sheet material are avoided. Ofcourse, the holes are positioned such that they do not extend to thecavity.

In a highly preferred embodiment the cover comprises a tuning openingfor deformation of the conductive sheet material. In the case of anon-elastic deformation, a tool can be inserted into the opening fordeforming the sheet material and the tuning opening can be closedafterwards. In the case of an elastic deformation, a tool may beinserted permanently in the tuning opening for maintaining a constantpressure on the sheet material, or e.g. a temperature-dependent pressureto compensate for (or cause a desired) temperature drift in theresonator.

In a further preferred embodiment the cover and the resonator body aremade of the same material. In this way, relative thermal movementsbetween the cover and the resonator body are avoided which could causee.g. sudden phase jumps.

In a further embodiment the resonator body comprises at least twocentering pins. The centering pins are necessary to assure that thetuning opening is correctly positioned for tuning the resonator. For acylindrical cavity, for example, the tuning opening can but need not bepositioned along the center axis of the cylinder, determined by thedesired tuning sensitivity and the mode of the electromagneticresonance. Depending on the design, the resonance frequency may increaseor decrease with increasing tuning force.

In a preferred embodiment the resonator body is made of a die cast part.Using a die cast part instead of a milled block for forming theresonator body, manufacturing costs are reduced. Preferably, the coveris also made of a die cast part.

The invention is also realized in a method for tuning a RF-resonator asdescribed above, comprising the following steps: measuring the resonancefrequency of the resonant cavity, and deforming the conductive sheetmaterial by applying an increasing mechanical force to the conductivesheet material until the target frequency of the resonant cavity isreached. In the case of a non-elastic (irreversible) deformation of thesheet material, the mechanical force must not exceed the value necessaryfor reaching the target frequency. In the case of an elasticdeformation, it is possible to exceed this force value and subsequentlydecrease the mechanical force for adjusting the resonator frequency tothe target frequency.

Further advantages can be extracted from the description and theenclosed drawing. The features mentioned above and below can be used inaccordance with the invention either individually or collectively in anycombination. The embodiments mentioned are not to be understood asexhaustive enumeration but rather have exemplary character for thedescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is shown in the drawing.

FIG. 1 a shows a resonator body with a resonant cavity being part of anembodiment of the inventive RF-resonator in a perspective view,

FIG. 1 b shows the resonator body of FIG. 1 a covered by a thin metalfoil, and

FIG. 1 c shows a cover mounted on the resonator body of FIG. 1 b, thecover having a tuning opening for deforming the metal foil.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a to FIG. 1 c three steps of assembling a prototypeRF-resonator 1 are shown. With reference to FIG. 1 a, a rectangularmilled block forms a resonator body 2 which contains a centeredcylindrical hole as a resonant cavity 3. To the left and to the right ofthe cavity 3, two rectangular holes 4, 5 are formed in the resonatorbody 2 for receiving a first and a second connector (not shown) beinginserted through two pinholes from the bottom of the resonator body 2.Two centering pinholes 6, 7 and ten threaded holes 8 are formed on theresonator body 2 as well.

With reference now to FIG. 1 b, the top face of the resonator body 2 ofFIG. 1 a is covered with a thin metal foil 9 which has holes at thepositions of the pinholes 6, 7 and the threaded holes 8. The metal foil9 covers the resonant cavity 3 and is fixed by mounting a thick cover 10on top of the resonator body 2, as shown in FIG. 1 c. The two pinholes6, 7 and ten threaded holes 8 formed in the resonator body 2 arecontinued through the cover 3. Thus, the cover 3 may be fixed to theresonator body 2 by inserting mounting screws into the threaded holes 8.In such a way, the metal foil 9 is firmly fixed between the resonatorbody 2 and the cover 3, such that unwanted deformations of the metalfoil 9 leading to changes in the center frequency of the resonant cavity3 are avoided.

A tuning opening 11 is provided in the center of the cover 10. Thetuning opening 11 is centered by two centering pins (not shown) beinginserted into the two pinholes 6, 7. The tuning opening 11 should bepositioned such that a reasonable force/movement to frequency dependencyof the resonance is achieved. This depends on the mode of theelectromagnetic field in the cavity. Shown in the example is a tuningopening exactly on the center axis of the cylindrical cavity 3 forhaving a small influence on the electromagnetic field inside of thecavity 3 for the mode chosen, just sufficient to compensate formanufacturing tolerances.

Tuning of the RF-resonator 1 is typically performed by tool insertion inthe minor tuning opening 11 of the cover 3 with increasing force until asufficient non-elastic deformation of the metal foil 9 is reached sothat the desired center frequency of the RF-resonator 1 is attained.During this process, the frequency of the resonant cavity 3 has to beobserved. Alternatively, it is possible to use an elastically deformablefoil and to fix the tool inside of the tuning opening when the targetfrequency is reached.

In summary, the prototype RF-resonator 1 can be easily manufactured andhence production costs are minimized. There are no ‘exotic components’being used for the frequency tuning of the RF-resonator 1, a smallnumber of parts is needed, and all parts (resonator body, cover, sheetmaterial) can be manufactured from the same material to avoid relativethermal movements (causing e.g. sudden phase jumps). The RF-resonator 1is also suitable for high-volume mass production, especially if themilled block forming the resonator body 2 is replaced with a die castpart. Screws may also be replaced by a suitable soldering process.Furthermore, no precision parts except the resonator cavity itself areneeded.

The performance of the RF-resonator 1 has been verified with high Qworking design at 85 GHz, but a much higher practical frequency limitbeyond 200 GHz is expected. Furthermore, long-term stability of theRF-resonator 1 is expected to be superior to traditional resonatordesign, especially concerning robustness in vibrating environments.

1. RF-resonator, comprising: a resonator body with at least one resonantcavity, and a conductive sheet material forming a surface area of theresonant cavity, wherein the conductive sheet material isnon-elastically deformable by application of a mechanical force fortuning the RF-resonator to a target frequency.
 2. RF-resonator accordingto claim 1, wherein the conductive sheet material is a metal foil. 3.RF-resonator according to claim 1, wherein a face of the resonator bodyis covered by the conductive sheet material.
 4. RF-resonator accordingto claim 3, wherein the conductive sheet material is fixed to theresonator body by a cover mounted on the resonator body.
 5. RF-resonatoraccording to claim 3, wherein threaded holes are formed in the cover andthe resonator body for mounting the cover on the resonator body. 6.RF-resonator according to claim 3, wherein the cover comprises a tuningopening for deformation of the conductive sheet material. 7.RF-resonator according to claim 6, wherein the cover comprises at leasttwo centering pins.
 8. RF-resonator according to claim 3, wherein thecover and the resonator body are made of the same material. 9.RF-resonator according to claim 1, wherein the resonator body is made ofa die cast part.
 10. Method for tuning a RF-resonator comprising: aresonator body with at least one resonant cavity, and a conductive sheetmaterial forming a surface area of the resonant cavity, wherein theconductive sheet material is non-elastically deformable by applicationof a mechanical force for tuning the RF-resonator to a target frequency,said method comprising the following steps of: measuring the resonancefrequency of the resonant cavity, and deforming the conductive sheetmaterial non-elastically by applying an increasing mechanical force tothe conductive sheet material until the target frequency of the resonantcavity is reached.